1. Field of the Invention
The present invention relates to methods and apparatus for pulling a single crystal and, more particularly, to methods for pulling a single crystal wherein a single crystal of silicon or the like is pulled by a pulling method such as the Czochralski method (hereinafter, referred to as the CZ method), and an apparatus for pulling a single crystal.
2. Description of the Relevant Art
At present, the majority of silicon single crystals (ingots) used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like have been pulled by the CZ method. FIG. 1 is a sectional view of a conventional apparatus for pulling a single crystal using the CZ method, and in the figure, reference numeral 11 represents a crucible.
The crucible 11 comprises a bottomed cylindrical quartz crucible 11a and a bottomed cylindrical graphite crucible 11b fitted on the outer side of the quartz crucible 11a. The crucible 11 is supported with a support shaft 18 which rotates in the direction shown by the arrow A in the figure at a prescribed speed. A heater 12 of a resistance heating type and a heat insulating mold 17, arranged around the heater 12, are concentrically arranged around the crucible 11. The crucible 11 is charged with a melt 13 of a material for forming a crystal which is melted by the heater 12. On the central axis of the crucible 11, a pulling axis 14 made of a pulling rod or wire, which is suspended, and at the front of the pulling rod or wire, a seed crystal 15 is held by a holder 14a. These parts are arranged within a water cooled type chamber 19 wherein pressure of the chamber can be controlled.
A method for pulling a single crystal 16 using the above-mentioned apparatus for pulling a single crystal is described below by reference to FIGS. 1 and 2. FIGS. 2(a)-(d) are partially enlarged front views diagrammatically showing the seed crystal 15 and the steps in a conventional method for pulling a single crystal.
Although it is not shown in FIG. 1, an electric current is applied to the heater 12 so as to melt the material for forming a crystal. after reducing the pressure in the chamber 19. Then, an inert gas is introduced into the chamber 19 so as to make an inert gas atmosphere at a prescribed pressure within the chamber 19.
While the pulling axis 14 is rotated on the same axis in the reverse direction of the support shaft 18 at a prescribed speed, the seed crystal 15, held by the holder 14a, is descended and is brought into contact with the melt 13 so as to make the front portion 15a of the seed crystal 15 partially melt into the melt 13. Then, the pulling of the single crystal 16 from the melt 13 is started. This is referred to as the seeding as shown in (FIG. 2(a).
In making a crystal grow at the front portion 15a of the seed crystal 15, the pulling axis 14 is pulled at a higher speed than the below-described pulling speed in the formation of a main body 16c. The crystal is narrowed to have a prescribed diameter, leading to the formation of a neck 16a. This is referred to as the necking step (FIG. 2(b)).
By slowing down the pulling speed of the pulling axis 14 (hereinafter, simply referred to as the pulling speed), the neck 16a is made to grow to have a prescribed diameter, leading to the formation of a shoulder 16b (FIG. 2(c)).
By pulling the pulling axis 14 at a fixed rate, the main body 16c having a uniform diameter and a prescribed length is formed (FIG. 2(d)).
Although it is not shown in FIG. 2, in order to prevent induction of high density dislocation to the single crystal 16 by a sudden temperature change when the separation of the single crystal 16 from the melt 13 approaches, the diameter of the single crystal 16 is gradually decreased so that the temperature of the whole single crystal 16 is gradually lowered, leading to the formation of an end-cone. Next, the single crystal 16 is separated from the melt 13. Finally, the single crystal 16 is cooled at the end of the pulling of the single crystal 16.
One of the important steps in the pulling of the single crystal 16 is the above-mentioned necking step called the Dash method (J. Appl. Phys. 30 [4] (1959) W. C. Dash. p.459-473) (FIG. 2(b)).
The object of the necking step is described below. In the above seeding step (FIG. 2(a)), the front portion 15a of the seed crystal 15 is preheated to some extent and is brought into contact with the melt 13. Ordinarily, there is a difference of 100.degree. C. or more between the preheating temperature (about 1300.degree. C. and less) and the melting point of the seed crystal 15 (about 1410.degree. C.). Therefore, in contact with the melt 13, the front portion 15a of the seed crystal 15 has a steep temperature gradient, leading to the induction of dislocation caused by a thermal stress thereto. It is necessary to make the single crystal 16 grow after excluding the dislocation which propagates and inhibits single crystal growth. Since the dislocation generally tends to grow in the vertical direction to the growth interface of the single crystal 16, the shape of the growth interface (the front plane of the neck 16a) is made downward convex, so as to exclude the dislocation outward.
In the pulling of a single crystal, the faster the pulling speed, the smaller the diameter of the single crystal, or the more downwardly convex the shape of the growth interface of the single crystal. Therefore, in the above necking step, it is desired that the pulling speed be made as fast as possible to make the diameter of the neck 16a smaller, or to make the shape of the growth interface more downwardly convex, so as to efficiently exclude the dislocation outward.
In the above conventional method for pulling a single crystal, the seed crystal 15 having a diameter of, for example about 12 mm has been generally used in order to pull the single crystal 16 having a diameter of about 6 inches and a weight of 80 kg or so. In this case, the larger the diameter of the neck 16a is, the more safely the single crystal 16 can be supported, while the smaller the diameter of the neck 16a is, the more efficiently the dislocation can be excluded. In order to meet both of the requirements, the neck 16a having a diameter of 3 mm or so is selected.
Recently, however, in order to produce a more highly integrated semiconductor device at a lower cost and more efficiently, the wafer has been required to have a larger diameter. Now, for example, the production of the single crystal 16 having a diameter of about 12 inches (300 mm) and a weight of 300 kg or so is desired. When the diameter of the single crystal 16 is made larger, the weight of the shoulder 16b band tail inevitably becomes heavier. It becomes necessary to lengthen the main body 16c which can form a product in order to obtain the profitable yield. In other words, it becomes necessary to grow a heavy single crystal.
When the requirement is satisfied, the neck 16a having a conventional diameter (usually 3 mm or so) cannot withstand the weight of the pulled single crystal 16 and breaks, resulting in the falling of the single crystal 16.
In growing the above heavy single crystal 16, the diameter of the neck 16a needs to be about 5 mm or more in order to prevent the occurrence of troubles such as a fall of the single crystal 16 and to pull the single crystal 16 safely, which is calculated from the silicon strength (about 16 kgf/mm.sup.2). However, when the diameter of the neck 16a is 5 mm or more, the dislocation which is induced in contact of the seed crystal 15 with the melt 13 cannot be sufficiently excluded outward.
In order to solve the problem, a method for growing a heavy single crystal was proposed in Japanese Kokai No. 62-288191, wherein the diameter is once increased after growing the neck 16a, and is reduced and is increased again, so as to form a high-strength holding portion having a large diameter. which is mechanically held. It is possible to hold the heavy single crystal by this method, but a special jig, control, and the like which are exclusive to the mechanical holding are required in order to perform the mechanical holding in the method. In addition, when the mechanical holding is conducted on the high-strength holding portion, there is a possibility that shaking or the like is given to the high-strength holding portion, so that the growing portion is caused to have dislocation by the shaking or the like. As a result, there is a probability that the yield of the product is lowered. One of the present inventors invented a method for pulling a single crystal 16, wherein by irradiating the front portion 15a of a seed crystal 15 with a laserbeam or the like from a laser beam generator or the like, the temperature of the front portion 15a of the seed crystal 15 is gradually raised so as to be almost the same temperature as that of a melt 13, and then, the seed crystal 15 is brought into contact with the melt 13, and the single crystal 16 is pulled from the melt 13 without forming a neck 16a (Japanese Patent Application No. 08-43765). In this method, since the temperature of the front portion 15a is adjusted to be made close to that of the melt 13 before the seed crystal 15 is brought into contact with the melt 13, a sudden change in temperature (thermal shock) caused by the contact with the melt 13 can be reduced and the number of induced dislocations can be decreased. Therefore, even if the neck 16a is not formed, the single crystal 16 can be pulled with a decreased number of the induced dislocations, and the single crystal 16 heavier than before can be pulled.
However, since the irradiation of the laser beam is usually conducted only from one direction, the seed crystal 15 can be heated only from one direction. so that it is difficult to uniformly heat the front portion 15a of the seed crystal 15. As a result, it is difficult to perfectly exclude the thermal shock which affects the front portion 15a in contact of the seed crystal 15 with the melt 13, so that it is difficult to perfectly inhibit the induction of the dislocations to the single crystal 16.